Silence Will Eat

I apologise for the scruffiness of this cartoon. I thought of/drew/coloured this in literally five minutes as I'm about to go out and I wanted to get the idea down before I forgot it. I've just watched the jaw-droppingly fantastic Doctor Who episode 'Day of the Moon' (likely now my favourite Doctor Who episode, and that includes the classic series), featuring the terrifying Silence. They're a species with a clever trick: the minute you look away from them you forget everything about them, even that you saw them in the first place. They've been on our planet since "The wheel and the fire" apparently - so quite a while then - but how on earth did they find stuff to eat? Maybe this cartoon will help explain...